Soak spinach in cold water and add 2 tbsp of vinegar. Leave spinach in the water for 20 minutes. Then drain it. Wash and drain it one more time.In a large pot, take vegetable it and beef. Stir and cook until it absorbs its own juice over medium heat. Add chopped onion. Stir and cook until onion is tender. Add tomato paste, stir for 2-3 minutes. Chop spinach and add it to the pot. Stir until it loses its volume. Add 3/4 cup of boiling water, rice and black pepper to the pot and stir. Turn the heat down to low. Cover the lid. Simmer until the rice is cooked. Add salt.Serve it warm with plain yogurt.